A. Field Of The Invention
The present invention relates to a device suitable for use in an anterior fontanelle pressure sensing system. This device allows one to make rapid and accurate measurements of the anterior fontanelle pressure of an infant using a noninvasive technique.
B. Description Of The Related Art
The importance of measuring intracranial pressure in infants afflicted with certain pediatric disorders, such as Reye Syndrome, head injuries, hydrocephalus, and subarachnoid hemorrhage is being increasingly appreciated. However, invasive measurement techniques, for example, intraventricular catheters, subdural or epidural strain gauges, or telemetric devices are not well suited for routine monitoring of intracranial pressure in the infant. The first three create a potential for infection and the fourth would require major surgery to implant and remove the device.
Therefore, in recent years, considerable attention has been paid to the development of a noninvasive approach for evaluating intracranial pressure in newborns and infants. Instrumentation for estimating neonatal intracranial pressure has generally been designed to take advantage of the anterior fontanelle window of the newborn cranium. The anterior fontanelle is present at birth and closes between the ages of one and two years. The anterior fontanelle is a quadrangular defect bordered by the junction of the coronal, sagittal, and metopic sutures. Thus, in the face of increased intracranial pressure, the infant's brain may bulge into the anterior fontanelle and quantifications of this pressure increase is a noninvasive means of determining the magnitude of the pressure rise. Unfortunately, the devices of the prior art have met with limited acceptance because of a lack of consistent correlation to intracranial pressure; high sensitivity of transducer output to patient position and motion; lack of continuous monitoring capability in some designs; inability to correct for zero drift; the inability to calibrate in situ; and poor reproducibility following successive reapplications of the sensor to a given fontanelle.
The two devices that have been predominately used for anterior fontanelle pressure measurement are the aplanation transducer and the fiberoptic sensor.
The aplanation devices generally comprise a circular guard ring or baseplate (a plane surface) surrounding a pressure-sensitive plunger. When the guard ring or baseplate is placed against a membrane distended by pressure, the membrane will bulge through any hole in the surface When the plunger is placed in the hole and a force applied to it so that the end of the plunger is in the same plane as the baseplate, the pressure exerted by the plunger will theoretically be the same as that within the membrane Unfortunately, aplanation devices are typically motion sensitive, requiring minimal patient positional changes Also, these devices have no means for in situ calibration or baseline correction.
The fiberoptic sensor consists of a pressure sensitive sensor membrane with a miniature mirror mounted on its surface. Three fiberoptic columns are contained in a pneumatic tube connecting the sensor to a monitor The monitor transmits light to the sensor mirror via a central efferent optical fiber and compares the amount of reflected light carried back by the vertically flanking afferent fibers. A bellows systems can adjust the air pressure in the pneumatic tube in such a way that the sensor mirror is central and hence an equal amount of light is reflected back by the afferent fibers When the pressure acting on the contact surface of the sensor tilts the mirror, uneven reflection of light is fed back to the monitor. In response, the monitor balances the air pressure in the pneumatic tube to equalize the pressure on the transducer surface. The air pressure required to keep the mirror properly balanced is continuously displayed on the digital screen and a pen recorder simultaneously transcribes this pressure.
A disadvantage of this system is that sensor output is often affected by pressure with which the device is applied to the anterior fontanelle. Minimizing this problem requires meticulous attention to the application of the sensor, and reapplication of the sensor is required, often after only one or two hours.
Moreover, in most studies utilizing devices of the prior art to measure anterior fontanelle pressure, particularly the fiberoptic sensor, it appears that the sensor was applied to the fontanelle with prior knowledge of the intracranial pressure as measured by a ventricular catheter In such a case, it would often pose no problem to adjust the application force for good initial correlation. However, correlations between intracranial pressure and anterior fontanelle pressure following blind applications of either the fiberoptic sensor or aplanation devices have not been reported Therefore, usefulness of the prior art devices appears to be limited in that reproducible correlations between anterior fontanelle pressure and intracranial pressure upon blind application of the transducer have not been demonstrated. In addition, present designs have not provided any means by which transducer drift can be assessed in situ or baseline changes corrected following patient repositioning.
The problems enumerated in the foregoing are not intended to be exhaustive but rather are among many which tend to impair the effectiveness of devices presently employed to measure anterior fontanelle pressure in the infant. Although other noteworthy problems may also exist, those presented above demonstrate that the monitors used to monitor anterior fontanelle pressure have not been altogether satisfactory and illustrate that there is currently a great need for improved devices.